The invention relates to the field of fabricating integrated circuits and other electronic devices and in particular to an improved method of photoresist patterning that provides a wider process latitude and higher yield during the formation of dual damascene structures.
The manufacture of integrated circuits involves a lithographic process to form a pattern in a photoresist layer and one or more etching steps to transfer each pattern into a substrate. While most photoresists are optimized for front end of the line (FEOL) applications that generally have smaller feature sizes or critical dimensions (CD) than back end of the line (BEOL) applications, BEOL processes have unique challenges that require special solutions in some cases. For example, when forming a damascene structure, the lithography process usually must contend with considerable topography and has to be compatible with dielectric materials that can poison the photoresist.
There is an ever present demand for smaller circuits in semiconductor devices in order to achieve higher performance. From a photoresist standpoint, a smaller CD in a feature that will become a circuit is more easily printed when the film thickness is reduced or when the exposure wavelength is decreased according to the equation R=kxcex/NA. R is the minimum CD that can be resolved while k is a process constant, xcex is the exposure wavelength, and NA is the numerical aperture of the exposure tool. Thinner films help to lower k but in the example of a damascene process, the uneven surface of the substrate prevents a thin photoresist film from becoming planar. Film thickness non-uniformity leads to problems with controlling reflectivity off the substrate during exposure and as a result there is a wide variation in printed CD for each feature in the photoresist pattern. In addition, if photoresist fills the via part of the damascene structure, it is difficult to remove during the patterning step and residue or scum is often left behind that can increase cost due to rework and lower yield
A bilayer concept has been introduced to overcome the difficulties associated with imaging a thin single layer of photoresist. Typically, the top layer in a bilayer scheme is a thin film of photoresist containing a small percentage of an element like silicon that can easily form an etch resistant oxide in an oxygen plasma. The bottom layer is thicker so that it can form a planar surface over topography and often contains highly absorbing material that minimizes reflectivity to improve top layer patterning. The bottom layer is usually not photosensitive but may be comprised of a crosslinkable component that reacts with a thermally generated acid. The resulting crosslinked layer tends to be more unreactive toward the solvent or other components of the top layer to avoid mixing of the two layers. In theory, the thin photosensitive layer on a planar underlayer should provide a path to forming small feature sizes with a good process latitude during patterning of the top layer. The silicon containing film provides a high selectivity for transferring a pattern through a thick underlayer during an oxygen etch.
However, a lack of maturity in silicon containing photoresist technology has prevented widespread acceptance of the bilayer concept in the industry. When the composition of the silicon containing photoresist is optimized to provide more silicon content for better etch selectivity, the imaging performance usually suffers. CD non-uniformity across the wafer is one particular problem that is observed when imaging immature bilayer photoresists.
In order to achieve finer resolution in patterns, the exposing wavelength (xcex) has been steadily shifting lower in recent technology generations or nodes. For larger feature sizes from about 300 nm to about 1 micron, i-line (365 nm) or g-line (436 nm) exposure tools are more popular because of a lower cost of ownership. For the 130 nm to 250 nm nodes, Deep UV (248 nm) exposure tools are considered state of the art. Meanwhile, 193 nm exposure tools are thought to be the best solution for reaching the 100 nm node and a 157 nm technology is being developed for the 70 nm node.
With the shift to 248 nm and 193 nm exposure wavelengths, a new lithography concept was introduced in which photoresists operate by a chemical amplification (CA) mechanism whereby one molecule of strong acid is capable of causing hundreds of chemical reactions in an exposed film. A strong acid is generated by exposing a photosensitive component and the acid reacts with acid labile groups on a polymer in positive tone photoresist. In negative tone photoresist, the acid initiates a crosslinking reaction. Unfortunately, this new approach is quite sensitive to traces of base compounds such as airborne amines or amines that diffuse into the chemically amplified photoresist from an adjacent layer. Sometimes, amine concentrations as low as parts per billion (ppb) can inhibit or xe2x80x9cpoisonxe2x80x9d the CA reaction enough to prevent a pattern from being formed.
Another concern that has recently appeared for 193 nm and 248 nm applications is outgassing from silicon containing resists. High energy exposures are capable of breaking Cxe2x80x94Si or Oxe2x80x94Si bonds that can release volatile fragments of silicon containing material which can deposit on the lens component of the exposure system. Eventually, the deposit is converted to SiO2 that forms a permanent coating and irreversibly damages the expensive optics.
An option in the bilayer approach is to expose a silicon free photoresist layer over an underlayer and then selectively introduce a silicon reagent into either the exposed or unexposed regions. This method of treating a photosensitive film with a silicon compound that reacts to become incorporated into the film is called silylation. In U.S. Pat. No. 5,922,516, the underlayer is a photoresist that has been thermally crosslinked at a temperature between 110xc2x0 C. and 140xc2x0 and a silicon compound in a vapor phase reacts with the top photoresist. This method appears to be limited to an underlayer which is a 365 nm or 436 nm sensitive material because Deep UV (248 nm) and 193 nm photoresists are typically high activation energy systems that are normally processed up to 150xc2x0 C. without crosslinking. A lack of maturity in silylation tools is another concern for this technique.
A related bilayer method described in U.S. Pat. No. 5,286,607 involves silylation of the underlayer which is a photoresist and then patterning a second photoresist layer above the silylated material. Generally, silylation does not provide the same uniformity of silicon content across a film and into a film as when the coated layer already contains silicon. Moreover, an optimized bilayer system has the refractive index (RI) of the underlayer matched to the top layer so that the patterning process produces a vertical profile in the top layer. In other words, the sidewalls on the photoresist image form a straight line and do not have a foot at the interface with the underlayer. A foot is defined as a lateral extension from the base of a photoresist image where some photoresist remains on a substrate. When the (RI) of the two layers is not matched, a foot on the patterned image or photoresist residue on the underlayer tend to form which leads to problems in the subsequent etch transfer step. It is difficult to control the RI with a silylation technique because the composition as a function of depth is not uniform unlike the case with a film formed by spin coating a silicon containing material.
In another prior art example, U.S. Pat. No. 4,882,008 teaches how to perform a silylation and subsequent etch processes in the same chamber. However, the method does not deal with new challenges of photoresist poisoning by amines or with the unique challenges of patterning to form damascene structures.
U.S. Pat. No. 6,120,974 describes a novel polymer that forms an amine after a 193 nm patterned exposure and which forms a sulfonic acid following a second blanket exposure to neutralize the amine in first exposed regions. Regions that contain only sulfonic acid catalyze a reaction when a metal alkoxide is introduced and form a metal oxide which serves as an etch mask for the next step. Polymers with dual functionality are especially difficult to purify because traditional washing methods involving acid or base solutions will react with either the base precursor group or the acid precursor group. Small amounts of unreacted precursor polymer having acid or base groups can interfere with the precise compositional balance required to achieve a neutralization in double exposed regions and thereby cause unwanted oxide formation.
While some imaging methods have been proposed that bypass photoresist entirely such as in U.S. Pat. No. 5,700,628, they have not be accepted into manufacturing schemes as yet. In this case, 248 nm radiation is claimed to have a sufficient energy to break Sixe2x80x94F bonds in a fluorinated polysilicon substrate that is from 100 to 1000 Angstroms thick. However, the transparency of such a substrate is not high enough to allow light to penetrate very far below the surface and the exposure dose needed to break a sufficient number of Sixe2x80x94F bonds would be quite high such that the throughput is not compatible with a low cost process.
Although the underlayer with a thickness of about 5000 to 15000 Angstroms in a bilayer scheme can be used as an anti-reflective layer to absorb light that passes through the top layer, other anti-reflective coatings (ARCs) are commercially available and generally are used as a much thinner coating in the range of about 500 to 1000 Angstroms. These thin ARCs are employed when the substrate is relatively flat and have been tuned so that their RI matches a particular family of photoresists. For example, one type of ARC is available for i-line photoresists and another type has been developed for imaging Deep UV photoresists. Once the photoresist pattern is formed, it must be transferred with an etch process through the ARC. Usually, the etch selectivity is poor when a non-silicon containing imaging layer is the etch mask and corner rounding occurs as shown in U.S. Pat. No. 6,080,678. A new etch gas mixture including O2 and SO2 is claimed to provide less corner loss on the photoresist image.
A prior art method of forming a damascene structure is shown in FIGS. 1a-1c. In FIG. 1a, an opening 18 has been formed in photoresist 17 on a stack consisting of passivation layers 12 and 16, dielectric layers 13 and 15, etch stop layer 14, metal layer 11, and substrate 10. The opening 18 is etch transferred through underlying layers 13, 14, 15, and 16 to form a via hole 22. After photoresist 17 is stripped, a second photoresist 19 is coated and patterned to form a trench opening 20 in FIG. 1b. Opening 20 is etch transferred through passivation layer 16 and dielectric layer 15. Photoresist 19 is stripped and a metal layer 21 comprised of copper or aluminum is deposited to fill via and trench openings. After a planarization step, the completed damascene structure appears as illustrated in FIG. 1c. 
In a damascene patterning process described in U.S. Pat. No. 6,319,809, a UV radiation step is used to treat the trench and via openings. This treatment reduces the amount of photoresist poisoning that occurs due to diffusion of trace amounts of amines from the dielectric layers. However, this method cannot completely eliminate photoresist poisoning that is depicted in FIG. 2. In FIG. 2, a chemically amplified resist 23 is coated to fill a via hole and form a layer on the damascene structure. However, poisoning from one or more layers 12, 13, 14, 15, or 16 has inhibited the chemical reaction initiated by exposure in region 21. As a result, only the top portion of photoresist 23 in region 21 has been converted to a soluble material that is washed away in developer. A rework process is required to strip the photoresist 23 and repeat the coating and imaging steps. An improved method is needed that totally avoids contamination of the photoresist from dielectrics or from other layers such as nitrides that may be employed as passivation layers in the damascene process.
Another defect that occurs in damascene patterning is depicted in FIG. 3. Photoresist 25 has been exposed and developed to form via hole 24. However, the action of the developer has been restricted somewhat such that some photoresist residue 26 remains in corners of via hole 24. This behavior is possible with all types of photoresist and causes additional processing and costly rework before a clean via 24 can be achieved and metal deposition can proceed. Therefore, it is desirable to implement a method that does not involve coating photoresist in via holes.
It would also be desirable to separate the imaging component from the etch resistant characteristic of the silicon containing layer in the bilayer scheme. Ideally, an improved method should be compatible with a low cost, high throughput manufacturing process and should be applicable to a variety of exposing wavelengths, especially those that are preferred for forming feature sizes below 250 nm. Preferably, the method should be readily implemented into manufacturing so that lengthy times required to qualify new materials can be avoided.
An objective of the present invention is to provide an improved method of forming damascene structures by optimizing the photoresist patterning process so that it is resistant to poisoning from adjacent layers and does not form scum or residue in via holes during trench patterning.
A further objective of the present invention is to separate the photosensitive component in the patterning process from the silicon containing layer that provides high etch selectivity for pattern transfer.
A still further objective of the present invention is to provide a lithography method that is compatible with a wide variety of exposure tools that are currently available in manufacturing such as g-line (436 nm), i-line (365 nm), Deep UV (248 nm), and ArF (193 nm) steppers and scanners. The method should also be extendable to the next generation of sub-200 nm exposure systems.
A still further objective of the present invention is to provide a photoresist patterning method that is compatible with a high throughput, low cost manufacturing process.
These objectives are achieved by modifying the traditional bilayer approach and removing the requirement that the top layer is both photosensitive and silicon containing. This method is especially useful for the trench patterning step following a process that has formed via holes. In one embodiment, a resin layer is coated on a damascene stack on a substrate consisting of at least one passivation layer, at least one dielectric layer, and one or more via holes formed in the stack. The resin layer is etched back until it is coplanar with the top layer of the damascene stack and fills only the via holes. Next an organic underlayer material such as a Novolak resin, an i-line photoresist or a commercially available underlayer composition is coated on the substrate and baked to form a planar layer. A silicon containing spin-on material that is not photosensitive such as SiO2 is then coated on the underlayer. This layer contains enough silicon to provide a high oxygen etch selectivity for transferring a pattern through the thick underlayer. An optional anti-reflective coating (ARC) is then formed on the silicon layer. A top layer is coated which is a state of the art photoresist that is commercially available and provides the minimum CD for the trench opening. The underlayer and resin plug protect the photoresist from poisoning that causes scum.
After a pattern which contains a trench opening is formed in the top photoresist layer, the pattern is transferred through the ARC and the silicon containing layer with a plasma etch. The etch gas is switched to oxygen and the pattern is then transferred through the thick organic underlayer and part way through the resin layer in the via holes. The etch gas is then switched again to selectively remove dielectric layer exposed by the trench opening. The dielectric layer is lowered to a level that is coplanar with the resin material in the via holes. The remaining resin in the vias is removed by an etch and the remaining underlayer is stripped. Then a metal such as copper is deposited in the trench and vias. A planarizing step such as a chemical mechanical polish lowers the metal until it is coplanar with the dielectric layer.
In a second embodiment, the ARC is omitted and the silicon containing layer may also be modified to contain a dye to control reflectivity during patterning of the top photoresist layer. The refractive indices of the silicon layer are adjusted to optimize the photoresist imaging process such that a foot is not formed on the photoresist sidewalls and no residue or scum occurs in exposed regions.
In a third embodiment, the thick organic underlayer fills the via holes in the damascene structure and also forms a planar layer upon which the silicon containing layer is formed. The ARC is optional and the top layer is a mature, state of the art photoresist that has good process latitude and achieves minimum feature size for a particular exposure wavelength. The process flow is similar to the first embodiment except that the silicon layer is an etch mask while the entire thickness of underlayer exposed by the photoresist pattern is removed including underlayer in via holes. The trench is then formed with another etch step. The remaining silicon containing layer and underlayer are stripped and metal is deposited to complete the damascene structure.